By far and away most people choose a used HDT that is equipped with a semi-automatic or fully automated transmission.
Eaton-Fuller Autoshift (1998-current)
If your looking at a 2003 or older HDT for your conversion then your choices are pretty much limited to an Eaton-Fuller Generation I or Generation II AutoShift semi-automatic transmission. HDT's that are equipped with an AutoShift transmission have a third pedal located to the left of the brake pedal which is depressed when coming to a complete stop and for initial starts from red lights, stop signs and so forth. Once you release the pedal at take off the transmission is fully automated from that point on until you need to come to a complete stop again at which time you'll need to depress the pedal again.
The Eaton Autoshift is a 10 speed transmission that sequentially shifts thru the gears from the selected starting gear. Example: 2nd-3rd-4th-5th-6th-7th-8th-9th-10th. The selected starting gear is chosen at start-up and remembered until the ignition is turned off. Depending on your differential gearing, you will usually start in 2nd or 3rd gear depending on you load. Some transmission computers are limited to starting in 3rd gear as the highest. Some Eaton AS -Fuller transmissions have two reverse gears.
Generation I Easton-Fuller AS transmission have a floor mounted pylon with a shift handle to select Reverse, Neutral, Drive, Hold, Low.&Generation II AS transmissions have a touch panel control module mounted on the dash or drivers seat.
Arvin-Meritor Freedomline (2001-2005)
Eaton-Fuller Ultrashift (2005-current)
Starting sometime in late 2003, early 2004 fully automated transmissions became available in HDT's with the launch of the Eaton UltraShift and the ArvinMeritor Freedomline. From what I've read there are pro's and con's with either transmission therefore it's advisable that you test drive both before purchasing an HDT with one or the other.
The Eaton UltraShift comes in 10 and 13-speed versions for the normal line haul trucks with an 18-speed for the larger 600+ horsepower engines.
If your looking to purchase a truck with a fully automated transmission then here is a Freedomline Vs Ultrashift discussion by several members of their opinions about both transmissions.
Volvo I-Shift (2008-current)
Volvo has since come out with their own fully automated 12-speed transmission that they call the I-Shift which is said to be the Cadillac of fully automated transmissions, however, the trucks that are equipped with an I-Shift are generally to expensive for the average Joe looking for a used HDT to convert to an RV hauler.
If you can hold out for a couple more years before making your purchase I-Shift equipped trucks will hit the used truck market....